The Washington massacre gunman had serious mental problems and suffered fits of rage, but was still able to hold an official security clearance for the complex where he killed 12 people.

Law enforcement officials told AP news agency that Aaron Alexis had paranoia and a sleep disorder and was hearing voices in his head.

Alexis motive in the Navy Yard rampage still remains a mystery.

They also said there has been no connection to international or domestic terrorism, and investigators have found no manifesto or other writings suggesting a political or religious motivation.

The attack was unlikely to lead to tighter gun controls. Measures proposed during national outrage over a school shooting in December that killed 20 children failed this year in Congress. It was at least the seventh mass shooting of his Barack Obama’s presidency.

Family members said that Alexis, 34, who was shot dead by police, had been treated for his mental problems since August.

The Navy had not declared its defence contract employee mentally unfit, which would have rescinded the security clearance that Alexis had from his earlier time in the Navy Reserves.

Alexis used a valid pass to get into the Washington Navy Yard. In the past, he had complained about the Navy and being a victim of discrimination and had several incidents with law enforcement, including two shootings

Alexis carried three weapons in the attack: an AR-15 assault rifle, a shotgun, and a handgun that he took from a police officer at the scene. Police said they were convinced Alexis had acted alone.

At the time of the shooting, Alexis was an employee with a company that was a Defence Department subcontractor on a Navy-Marine Corps computer project. He had access to the Navy Yard as a defence contractor and used a valid pass to enter the complex where more than 18,000 people work.

He had been a full-time Navy reservist from 2007 to early 2011, and had a string of misconduct problems but received an honourable discharge.

A convert to Buddhism who grew up in New York City, Alexis had had shooting incidents in 2004 and 2010 in Fort Worth and Seattle and was portrayed in police reports as seething with anger.